The present invention generally relates to a reinforced composite structure. More particularly, the invention is directed to a composite structure which is reinforced and assembled using composite thread. The composite thread, by holding together the formed details, allow assembly of the composite structure while in a staged condition such that the entire structure can be co-cured (curing of each member of the structure at the same time) and bonded in one operation. In addition, by strategically sewing the stitches of composite thread in the structure, peel forces on the structural joint are minimized.
Usage of composite materials in structures has been increasing rapidly. This is particularly true in the aerospace industry where the lightweight, high-strength characteristics of these materials are particularly desirable. In this art, strong, lightweight, tough, self-sustaining sheet material has been developed, these materials being composed as a class of a resinous sheet reinforced with layers of continuous, lineally aligned, parallel filaments. These sheets may be formed as a single layer sheet or as a multi-layer laminates, and thereafter thermoset to tough, hard, exceptionally strong panels, or skins for aircraft and the like.
As initially formed, these sheet materials are flexible and deformable, providing panel-forming members which can be draped or othewise conformed to various shapes and thereafter cured, by thermosetting, upon the application of heat and pressure thereto, to tough, strong skins or panels of permanent shape retention and having exceptional tensile strength imparted by the continuous filament reinforcing.
Panels or skins made up of these materials are exceptionally strong, lightweight structural units. However, structures made by the inclusion of reinforcing ribs or the like on such panels have heretofore seen limited applicability. Presently, these skins or panels are typically joined by first curing the sheet material into the desired shape and adhesively bonding a web or rib structure to the panel through the medium of a thermosetting adhesive.
However, as previously stated, uncured composite structure is flexible and deformable and when curing large combined structural members, such as an aircraft wing box, difficulties have been encountered in holding the parts in place, especially when transporting the structure to an autoclave or oven. Further, it has been found that tension loads, at unacceptably low values, on the cured panel or skin of the composite structure result in "peel" forces which tend to separate the web and panel of the structure at their interface (joint). cl PRIOR ART STATEMENT
Due to the formable nature of uncured composite laminated sheets, the current practice in fabricating a structure has been to individually cure the elements of the structure prior to assembly. A further bonding or fastening operation is necessary once the structure is assembled. Various attempts have been made to solve the peeling problem in rib cap to panel bonded joints under tension load. Thus, metal rivets and bolts have been used as fasteners at the interface of the composite rib/sheet joints. Unfortunately, this has increased the weight of the resulting structure because of requiring more plies of material due to the reduced laminate strength resulting from the necessary drilled fastener holes, increased fabrication time, and presented sealing problems where the panel forms part of a container, such as a fuel tank. Imbedding a metal "T" element into a joint has also been utilized. This has also been found to be unsatisfactory in that the element is expensive, increases the weight of the structure, requires a critical priming operation, is incompatible with the surrounding material due to its higher thermal co-efficient of expansion, and is too rigid, thereby resulting in a notch effect.
It has now been found that composite thread according to the present invention can be easily inserted into a staged composite structure with minimal disturbance to the fibers. By pulling the threads taut, the elements of the staged structure are held together. With bolts or rivets, holes must be drilled in the elements of the structure which cuts the composite fibers, and are subject to the above noted disadvantages. Further, there is normally no access to install corresponding fastener nuts (with of course the additional configuration problems due protrusion of the nut). Moreover, the composite elements are too soft, even in the staged condition, for efficient hole drilling and for the bolts to be effectively grip the material. It has also been found that composite thread according to the present invention is cheaper and lighter than bolts or rivets, can be stitched into the structure much quicker, does not waste material in that fastener holes are not required (though holes are required for the thread to pass through, their size is quite small), and results in only minimal disturbance to the composite fibers of the structure when sewn. In addition, by positioning the stitches of composite thread strategically, the strength of the structure is increased.